The invention relates to copolymers of tetrafluoroethylene copolymerized with (perfluorohexyl)ethylene and higher homologs, produced by aqueous dispersion polymerization.
Many prior patents disclose techniques for the dispersion polymerization of tetrafluoroethylene, and variations thereof. The dispersion polymerization of tetrafluoroethylene produces what has come to be known as “fine powder” resins. In such a process, sufficient dispersing agent is introduced into a water carrier such that, upon addition of tetrafluoroethylene in the presence of a suitable polymerization initiator and upon agitation, and under autogenous tetrafluoroethylene pressure of 10 to 40 kg/cm2, the polymerization proceeds until the target level of colloidally dispersed polymer particles is reached and the reaction is then stopped. See, e.g., U.S. Pat. No. 4,016,345 (Holmes, 1977).
Tetrafluoroethylene powders have also been produced by an alternative process of suspension polymerization, wherein tetrafluoroethylene monomers are polymerized in a highly agitated aqueous suspension in which little or no dispersing agent may be employed. The type of powder produced in suspension polymerization is termed “granular” resin, or “granular powder” . See, e.g., U.S. Pat. No. 3,655,611 (Mueller, 1972).
For both types of polymerization processes, copolymerization of tetrafluoroethylene with various fluorinated alkyl ethylene comonomers has been described. See, for example, U.S. Pat. No. 4,792,594 (Gangal, et al., 1988). The present invention relates to the aqueous dispersion polymerization technique wherein the product of the polymerization reaction is the copolymer of the invention dispersed within an aqueous colloidal dispersion. This process, generally, is one in which tetrafluoroethylene monomer is pressured into an autoclave containing water and certain polymerization initiators, along with paraffin wax to suppress coagulum formation, and an emulsifying agent. The reaction mixture is agitated and the polymerization is carried out at suitable temperatures and pressures. Polymerization results in the formation of an aqueous dispersion of polymer. The dispersed polymer particles may be coagulated by techniques known in the art to produce the polymer in the form of fine powder. When perfluorinated alkyl ethylene comonomers are introduced into the polymerization, it is known that the TFE reacts preferentially with the comonomer relative to TFE, and comonomer addition rate is important to the distribution of comonomer achieved in the copolymer. When this comonomer is added as a single precharge, the comonomer is found in polymerized form mostly in the core or interior of the polymer particles. The comonomer may also be injected through some or all of the polymerization process, and the injection sequence determines the structure of the shell.
Various prior patents have disclosed variations on techniques for the homopolymerization of tetrafluoroethylene and for the copolymerization of other monomers with tetrafluoroethylene. Among those are included U.S. Pat. No. 4,576,869 (Malhotra, 1986) and U.S. Pat. No. 6,177,533B1 (Jones, 2001). Within those references are contained certain procedures which have become, more or less, accepted procedures for determining certain defining and delineating properties associated with tetrafluoroethylene homopolymers and copolymers. Among those properties are:
(a) the Standard Specific Gravity (SSG), measured by water displacement of a standard molded test specimen, in accord with ASTM D-1457-90;
(b) the Raw Dispersion Particle Size (RDPS), determined by spectrophotometry or other suitable technique. See, e.g., U.S. Pat. Nos. 4,016,345 and 4,363,900. The measurements herein were obtained by laser light scattering using a Brookhaven 90 plus instrument;
(c) Resin surface area. The surface area of the dry coagulated resin is inversely related to RDPS. The measurements herein were obtained using a Coulter model SA3100 surface area analyzer using the BET method and Nitrogen as the absorbed gas.
In the cited prior patents, and almost universally, the SSG of a homopolymer specimen has come to define its molecular weight, with the relationship being inverse, that is, a high molecular weight (MW) corresponds to a low SSG and, generally, the lower the SSG, the higher is the molecular weight. Addition of comonomer into the polymerization process may also reduce SSG and, for resins modified with comonomer, SSG may be used to infer variations in molecular weight at a given constant comonomer level.
For tetrafluoroethylene fine powder polymers, generally, their RDPS's range from about 0.175 microns and below to about 0.325 microns. These fine powder resins are known to be useful in paste extrusion processes and in stretching (expansion) processes in which the paste-extruded extrudate, after removal of the extrusion aid lubricant, is stretched rapidly to produce porous, strong products of various cross-sectional shapes such as rods, filaments, sheets, tubes, etc. Such a stretching process is disclosed in U.S. Pat. No. 3,953,566 (Gore, 1976), assigned commonly with the instant invention. Products produced by this process are marketed under the well-known trademark GORE-TEX®.
Copolymers of tetrafluoroethylene and (perfluorobutyl)ethylene are disclosed in U.S. Pat. No. 6,541,589B1, also commonly assigned with the present invention. Therein, a copolymerization process and the products produced thereby are disclosed, including a PFBE copolymer which possesses small particle size coupled with high molecular weight, wherein the initiation of polymerization is by addition of permanganate, and the reaction is carried out in the absence of any ionic strength enhancer such as ZnCl2.
Still further, others have previously investigated the copolymerization of TFE with PFBE and higher homologs, and reported on the polymers produced thereby. For example, Kamiya (Japanese Patent Application 10-243976, filed August 28, 1998) discloses tetrafluoroethylene fine powder copolymers with (perfluoroethyl)ethylene (PFEE), (perfluorobutyl)ethylene (PFBE), and (perfluorooctyl)ethylene, with a stated objective including the manufacture of uniform, high-strength porous articles by stretching. Among others, the patentee concluded that, among these comonomers, the (perfluorobutyl)ethylene (PFBE) was preferred.
Prior to the discovery disclosed and claimed in the '589 patent, it was generally accepted that, for tetrafluoroethylene homopolymers and copolymers of the dispersion type, it was difficult to achieve a resin which combined both of the desirable properties of small particle size (RDPS) coupled with a high molecular weight (MW) (low SSG). Expressing the same conclusion in a different, equivalent way, it was generally accepted that a dispersion resin possessing a small raw dispersion particle size (RDPS) and a low standard specific gravity (SSG) was difficult or impossible to achieve. And seemingly small variations in SSG values, i.e. 2.160 to 2.157, reportedly produced significant variations in polymer properties. See, e.g., Jones, U.S. Pat. No. 6,177,533, wherein patentee specifically claims distinct embodiments wherein SSG values vary by 0.003 units. Moreover, the '589 patent, directed as it is to copolymers of TFE and PFBE, provided a breakthrough in achieving the oft-sought combination of small particle size and high molecular weight, namely, particle size below 0.203 micron and SSG of less than 2.143.
The present invention provides a dispersion type copolymer of tetrafluoroethylene and (perfluorohexyl) ethylene and higher homologous comonomers which possess a heretofore unachieved combination of both small fundamental resin particle size (RDPS) coupled with a low SSG (high MW). In addition, the new copolymers of the invention provide expanded products having extremely high, heretofore unachievable, tensile strength properties.